Missed that animation first time round Chris but it shows well the motion of the bucket. In the labelled CAD picture you showed a couple of posts ago is there a bit missing that keeps the boom rack in mesh with the gear - in some of the videos some of the shovals seemed to have a hoop over the boom to control the gear mesh. This bit;

That hold-down part is not part of the design for the Marion shovels. The one in that picture shows the dipper booms on the outside of the one piece solid main boom so they needed that upside-down U shape to connect everything. On the Marion shovels the main boom has a gap in the center that the dipper booms are in and the connector is in the center. Here is what the equivalent piece in the Marion looks like:It has the axle for the gears at the bottom and the plate at the top rides along the dipper boom. That presses the gear into the rack. Same end result but done with a part down the center rather than the outside. Here is another view of the Marion assembly, with the near-side parts removed to show the center:

Wrapping up the bearing blocks this afternoon - cut the top caps from some 3/8" square stock, drilled/tapped the holes to mount them to the bases, and drilled the axle bearing holes.Made up the bronze bearings, had them project out from the blocks slightly to give the gears something to rub against. With the axle in place to align the blocks to each other, held the blocks on top of the booms and marked where the vertical bolts came up - drilled/tapped matching holes in the blocks, plus an extra one at the end.Here is the two blocks bolted into the boom, using the end bolts and the four through bolts in the boom underneath. Things aligned pretty well, just needed one thin shim on one side to get the axle to line up freely.Then milled the caps down, just like on the previous ones, and bolted them down. The small gears needed a little trimming on the sander to get the whole assembly to fit between the bearing blocks. This photo shows the traveling yoke that the dipper boom slides through, holding the gear rack on the bottom of the boom in mesh with the small gears that move it - same as in the rendering from the 3D model in the previous post.And here is a family shot of all the parts:I'd like to put the booms together for a shot, but it is quite awkward to try and hold the main boom to slide in the dipper, then get all the bolts on the yoke shaft back in. I think that will wait a bit till I can get the turntable base made, all these parts are over 10 pounds so far and it gets tricky to handle them all together.

So it seems that the dipper can be moved to either side of the boom, and that's the reason for the two pulleys?

Not sure what you mean by either side? The two top pulleys route the chain above the boom and the crowd engine which will sit on top of the boom. The chain runs through the gap in the dipper boom as it goes in/out and pivots on the axle just added. The dipper boom only sits in the center space in the main boom, never to either side on the Marion shovels.

I have come up with a plan to make a temporary base for the boom so I can set it up, photos later today after its made...

Okay, the temporary base for the boom assembly is made, worked out quite well as a extra couple sets of hands for assembling the booms for testing. The bottom left block has a cross pin for the hole in the base caps, and the right block holds the main boom up at its 45 degree angle - both blocks are glued and screwed in from the bottom of the base since there is a lot of force on them.So, here is a shot with the dipper boom about halfway out:and with the dipper boom most of the way in:Shot from above showing how the dipper boom sits inside the main boom, and the hoist chain comes back down the center of the dipper handle:View from behind showing the gear rack, small drive gears, and the yoke block in the center that holds the dipper handle. Still to make are the two large gears that go on the outside ends of the axle.Front view of all that:You can see in the last shot how the wood core blocks are cut back to allow the dipper to swing from past straight down to above horizontal.

I better lock up the shop tonight, those shop elves will get drunk on a thinblefull of beer and start riding the bucket up and down!

My bad. Didn't realize the dipper was split down the middle from looking at your drawing.

Yeah, its hard to see what's connected to what with all the stacked up parts. I have a list of photo angles I want to get next time I go out to the real one, thousands of pics and still am missing details!